This application is based on application No. H11-178763 filed in Japan on Jun. 24, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a laser scanning apparatus for use in, for example, a laser printer.
2. Description of the Prior Art
Conventionally, in the field of laser scanning apparatuses, a technique has been proposed, for example in Japanese Patent Application Laid-Open H9-33850, whereby, of all the surfaces constituting a scanning lens, two are each so shaped that their radius of curvature as viewed on a sectional plane along the sub scanning direction varies independently of their shape in the main scanning direction. This makes it possible to properly correct curvature of field in the sub scanning direction and unevenness of optical magnification in the sub scanning direction. Another such technique is proposed in Japanese Patent Application Laid-Open H8-248308, which uses a single-element scanning lens having free-form surfaces on both sides. This also helps correct curvature of field.
However, the arrangement proposed in Japanese Patent Application Laid-Open H9-33850 mentioned above suffers from so-called groove-shaped aberration, which hampers proper focusing of a laser beam and thereby degrades the image formed. Moreover, this arrangement offers only such a degree of freedom in the design of the lens surface shapes in the sub scanning direction as to allow correction of curvature of field and unevenness of magnification in the sub scanning direction, and therefore does not allow satisfactory correction of groove-shaped aberration. As a result, in particular in cases where scanning is performed through a comparatively wide angle, it is not possible to achieve proper focusing of a laser beam at the periphery of the image formed.
Furthermore, in this arrangement, the lens elements constituting a scanning optical system are designed to be laterally symmetrical, and thus the lateral asymmetry inherent in a polygon mirror acts to degrade the overall optical performance. The lateral asymmetry inherent in a polygon mirror exerts a particularly marked effect when a scanning optical system is adapted for a wider scanning angle. In general, in a laser scanning optical system, the scanning width is determined as the focal length of the scanning optical system in the main scanning direction multiplied by the deflection angle. Thus, to obtain a given scanning width, the narrower the deflection angle a scanning optical system is designed to have, the longer its focal length needs to be made. This inevitably requires that the entire apparatus be made unduly large.
Similarly, also the arrangement proposed in Japanese Patent Application Laid-Open H8-248308 mentioned above suffers from groove-shaped aberration, which hampers proper focusing of a laser beam and thereby degrades the image formed. Moreover, in this arrangement, it is difficult to correct the unevenness of magnification in the sub scanning direction.
An object of the present invention is to provide a laser scanning apparatus that allows proper correction of curvature of field in the sub scanning direction, unevenness of optical magnification in the sub scanning direction, and groove-shaped aberration and that thus offers images of satisfactory quality.
Another object of the present invention is to provide a laser scanning apparatus that allows high-speed image formation by the use of a plurality of beams.
To achieve the above objects, according to one aspect of the present invention, a laser scanning apparatus is provided with: a light source device for emitting a laser beam; a deflector for deflecting the laser beam so as to scan a scanned surface with the laser beam; and a scanning optical system disposed in the optical path of the laser beam so as to focus the laser beam on the scanned surface. Here, the scanning optical system is composed of three or more lens elements, of which the lens element disposed closest to the deflector and the lens element disposed closest to the scanned surface each have a surface that is so shaped that the curvature thereof as viewed on a sectional plane along the sub scanning direction varies independently of the shape thereof in the main scanning direction.
According to another aspect of the present invention, a laser scanning apparatus is provided with: a light source device for emitting a laser beam; a deflector for deflecting the laser beam so as to scan a scanned surface with the laser beam; and a scanning optical system disposed in the optical path of the laser beam so as to focus the laser beam on the scanned surface. Here, the scanning optical system has at least three surfaces that are each so shaped that the curvature thereof as viewed on a sectional plane along the sub scanning direction varies independently of the shape thereof in the main scanning direction.
According to another aspect of the present invention, a laser scanning apparatus is provided with: a light source device for emitting a laser beam; a deflector for deflecting the laser beam so as to scan a scanned surface with the laser beam; and a scanning optical system disposed in the optical path of the laser beam so as to focus the laser beam on the scanned surface. Here, the scanning optical system has at least two optical components each having a surface that is so shaped that the curvature thereof as viewed on a sectional plane along the sub scanning direction varies independently of the shape thereof in the main scanning direction and laterally asymmetrically.